Vacuum device



arch 21, 1967 c. B. SIBLEY- VACUUM DEVICE Filed Feb. 11, 1965 O O IFlG.2

FIG?) United States Patent 3,310,226 VACUUM DEVICE Clifton B. Sibley,Needham, Mass., assignor to National Research Corporation, Cambridge,Mass, a corporation of Massachusetts Filed Feb. 11, 1965, Ser. No.431,949 9 Claims. Cl. 230-69) The present invention relates to vacuumpumping and particularly to the evacuation of vacuum systems by orbitingelectron vacuum pumps (hereinafter orbitron pumps). There have been manyattempts in the prior art to provide a convenient form of vacuum pumpingdevice utilizing ionization .and sorption effects. The orbitron pumpaccomplishes this without the direct use of magnets or cryogenics.

It is a principal object of this invention to provide an improvedorbitron pump construction.

It is a further object of this invention to provide an improvedtechnique of operating orbitron pumps.

The invention accordingly comprises the improved orbitron pump, theimproved technique of operating orbitron pumps, as embodied in processand machine form, the application of the technique to my improved pumpsand vacuum systems incorporating the above.

The invention is described below with reference to the accompanyingdrawings wherein:

FIG. 1 is a diagram of a vacuum system incorporating an orbitron pump;

FIG. 2 is an exploded partly sectional view of an improved orbitron pumpaccording to a first species of my invention;

FIG. 2A is .a sectional view of an assembled pump according to the samespecies as FIG. 2;

FIG. 3 is a sectional view of a portion of an orbitron pump according toanother species of my invention;

FIG. 4 is a sectional view of a portion of an orbitron pump according toa further feature of my invention.

Referring now to FIG. 1, there is shown an evaporator system. This is anexample of a vacuum syetem using the orbitron pump. A bell jar isconnected to a large (in comparison to the bell jar) diameter metalelbow 12 via sealed flanges 14. Several feedthroughs 16 are connected tothe flanges for admitting electric wires, gases, mechanical levers,etc., into the bell jar for control of the coating process. A titaniumfilament 18 is mounted on the elbow for reducing the pressure of thesystem to below the level of pressure wherein it is difiicult to startthe orbitron pump. The orbitron pump 20 is connected to the elbow sothat it has access to the bell jar 10 through a wide gas conductancepath.

Also connected to the elbow 12 are an electric power supply 22 forheating the titanium filament 18 and a tube 24. A rotary mechanical pump26 is connected to the tube via a valve 28. A cryosorption pump 30 isconnected to the tube via valve 32. A valve 34 is provided forconnection of a second cryosorption pump while the pump 30 is beingregenerated. A valved connection 36 is provided for connecting a leakdetector to the system. A tank 38 of inert gas is connected to the tubevia valve 40 for backfilling the system to raise its pressure.

Referring now to FIG. 2, the pump 20 is shown in exploded fashion. Thepump is made up of a flanged body 42 with cooling coils 44 welded to itsouter wall and a cover plate 46 mounting a filament 48 and a centralanode electrode 50. The electrode 50 includes a titanium slub 52. Thepump operation is based on emission of electrons from filament 48. Theelectrons enter a radial electric field between the anode 50 and thewall 42 which is grounded to serve as a cathode. The electrons spiralaround the anode 50 and eventually strike slug 52 providing a high heatinput to the slug.

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Titanium is sublimed from the heated slug and condenses on the cooledwall 42. Chemically active gases within the pump (oxygen, hydrogen,nitrogen, carbon dioxide, water vapor), tare gettered by the condensedtitanium. Inert gases in the pump are removed by the mechanism ofionization-collision of the gas molecules with electrons to formpositive ions which are attracted to the wall 42 and subsequently buriedby a fresh layer of titanium.

In prior art orbitron pumps the outer wall consists of a cylinder madeof the usual materials preferred for construction of high vacuumdevices. According to my invention, the usual wall construction ismodified by the provision of radial fins 50 secured to the wall 42 bybrazing, soldering or welding to give a good heat transfer connection.Clamping, for instance, is an unsatisfactory method of securing the finsto the Wall because of the high heat transfer impedance of a demountablejoint under conditions of high vacuum. The interior ends of the finsprovide a virtual annular surface for purposes of controlling theelectrostatic field of the pump. The fins may be made of the same Widthso that the virtual surface is cylindrical. The fins could be ofdifferent widths so that the virtual surface is elliptical or evenirregular. The fins can also be tilted so that they cross severaltransverse or longitudinal radial planes rather than each lying in atransverse radial plane as in the preferred embodiment of FIGS. 2 and2A.

Another species of my improved orbitron pump would be the use oftitanium washers (not shown) welded to the pump wall or a long spiral oftitanium, such as a lathe turning (shown as 350 in FIG. 3) welded to thepump wall.

The best mode of using my invention among the above species is thespecies of FIGS. 2-2A which offers higher gas conductance than theother.

A further and distinct improvement according to my invention is the useof tapered fins (450 in FIG. 4). Such a taper can be produced bymachining or electropolishing. The tapered form limits shadowing effects(with respect to the central source of titanium vapors) due to the useof the fins.

The invention is based on my observations of prior art orbitron pumps.These pumps tend to degrade in pumping speed characteristic after a longperiod of operation or after several pumpdown bake-out cycles. Iattribute this to flaking of titanium deposits on the pump wall. Theflakes have a poor thermal connection to the cooled pump wall andtherefore afford a poorer sticking factor for subsequent titaniumdeposits and pumped gas molecules and ions. It is believed that in someinstances the flakes tend to project randomly into the pump space anddisturb the electrostatic field, thereby upsetting the conditionsnecessary to maintain orbiting of electrons.

The above species of my invention provide a solution to this problem (a)because of the substantial increase in pump wall surface area affordedthereby and (b) because the fins are made of titanium, titanium depositswill adhere readily to the fins. The welding of the fins to the wallassures good conditions of heat transfer to enhance the stickingcoefiicient.

Referring again to FIG. 2A, the pump is indicated, as assembled, withconnection to power. There are two power supplies-a neon tube, highreactance transformer 60 for starting operations and a conventionalpower supply which may be DC. or AC. for running operation. Switches 62and 72 are provided in series with the power supplies.

Referring again to FIG. 1, the pump-down cycle of the vacuum system cannow be described. Initially, the system is at ambient pressure and allvalves are closed. Valve 28 is opened and pump 26 is operated to pumpthe system down to a pressure of about 10 microns Hg abs. or less. Thenvalve 32 is opened and sorption pump 30 is cooled to pump the systemdown to the 10' mm. Hg range. The elbow 12, bell jar and pump aresubjected to a mild bake (100-200" C.). Then, after baking, thesublimator 18 is operated to pump the system down to 5 l0- mm. Hg. Theorbitr-on is then turned on by closing the switch 72 (FIG. 2A) andturning on the filament heater power supply (not shown). It the pumpfails to take hold after a few minutes, then the filament power i out01f, switch 72 is opened and switch 62 is closed to establish an AC.glow discharge between the anode and cathode of the pump. Argon may bebled in from tank 38 via valve 40 to assist in starting the glowdischarge. After a few minutes the switch 62 can be opened to extinguishthe discharge, valves 40 and 32 are closed, sublimator 18 is againoperated to lower the pressure to 5 10 and the orbitron is re-started.Cooling water is admitted to the coils 44 of the pump. As the orbitronlowers the pressure, the emission current is gradually reduced foroptimum usage of the titanium sources within the pump A The abovedescribed glow discharge step improves adherence on the wall and finsand also activates the titanium surfaces within the pump for enhancedgettering action during the subsequent orbitron pumping step. Thisdischarge step should becarried out between one and ten minutes.

The fins 50 (FIGS. 2, 2A), are %21/S thick titanium having a width ofin., or about 4 of the pump diameter. The term diameter as used hereinrefers not only to the diameter of a cylindrical wall, but also to theaverage diameter of an elliptical or other form of annular pump wall.Typically, a 4 inch diameter pump would have about 40-50 fins /2 inch inwidth and spaced about /2 inch apart at the circumference of the pump.In general, the spacing between fins should be the same a their widthand the number of fins is thus determined by this relationship.

The limits of diameter for fin width correspond to efiectiveuse of thefins for the lower limit and minimal interference with orbiting electronoperation for the upper limit. The virtual surface, formed by the inneredges of fins of diameter width, corresponds to an equipotential line ofabout total anode to cathode bias. In this arrangement, the fins do notprevent operation of the pump.

positive bias on the central electrode so that the electrons spiralabout it, a source of reactive metal in the pump and reactive metal finssecured to the wall and extending radially inwardly toward the centralelectrode up to of the wall diameter, the fin surface area being atleast as great as the inner surface area of the wall.

2. The pump of claim 1 further comprising means for establishing a glowdischarge between the wall and central electrode.

3. Method of operating the pump of claim 2 comprising the steps offilling the pump with an inert gas to a pressure between 10" and 10torr, striking a glow discharge between the wall and central electrodein the presence of said inert gas and subsequently extinguishing theglow discharge and operating the pump with orbiting electron clouds.

4. A vacuum pump comprising, in combination, an annular wall, a centralanode electrode, at least one electron source for injecting electronsinto the space between the wall and central electrode, means formaintaining a positive bias on the central electrode so that theelectrons spiral about it, a source of reactive metal in the pump andreactive metal fins secured to the wall and extending inwardly towardthe central electrode, the fins having a width between and A pumpdiameter and a spacing between fins about the same as the average widthof the fins.

5. The pump of claim 4 wherein the fins are made of titanium.

6. The pump of claim 4 wherein the fins are pointed on their inner ends.

7. The pump of claim 4 wherein the fins are substantially vertical.

3. The pump of claim 4 wherein the fins are substantially horizontal.

9. The pump of claim 8 wherein the fins are formed of a continuousspiral.

References Cited by the Examiner UNITED STATES PATENTS 1,564,287 12/1925Smith 230-69 2,913,167 11/1959 Herb 23069 2,993,638 7/1961 Hall 230-693,070,719 12/1962 Jepsen 230-69 3,091,717 5/1963 Rutherford 230693,204,860 9/1965 Huber 230-69 3,236,442 2/1966 Davis 23069 FOREIGNPATENTS 908,487 10/1962 Great Britain.

LAURENCE V. EFNER, Primary Examiner.

1. A VACUUM PUMP, COMPRISING, IN COMBINATION, AN ANNULAR WALL, A CENTRAL ANODE ELECTRODE, AT LEAST ONE ELECTRON SOURCE FOR INJECTING ELECTRONS INTO THE SPACE BETWEEN THE WALL AND CENTRAL ELECTRODE, MEANS FOR MAINTAINING A POSITIVE BIAS ON THE CENTRAL ELECTRODE SO THAT THE ELECTRONS SPIRAL ABOUT IT, A SOURCE OF REACTIVE METAL IN THE PUMP AND REACTIVE METAL FINS SECURED TO THE WALL AND EXTENDING RADIALLY INWARDLY TOWARD THE CENTRAL ELECTRODE UP TO 1/8 OF THE WALL DIAMETER, THE FIN SURFACE AREA BEING AT LEAST AS GREAT AS THE INNER SURFACE AREA OF THE WALL. 